Chapter 13

Water and Its Solutions

13.1 Uniquely Water

• Water – common substance with uncommon properties

• Simple molecule – vital to life

*****Unique properties of H2O are due to three-dimensional arrangement and electron distribution

Water: The Molecular View

Substance

Formula

Molar mass

State Melting pt

Boiling pt

Methane CH4 16 g gas -183 -161

Ammonia NH3 17g gas -78 -33

Water H2O 18 g liquid 0 100

Nitrogen N2 28 g gas -210 -196

Oxygen O2 32 g gas -218 -183

• How is water different?

•liquid at room temperature•high melting/boiling pts•solid state LESS dense then liquid state

Water: The Molecular View• water is a polar molecule – electrons are unequally shared– Bent structure allows a positive pole

and a negative pole

Intermolecular Forces in Water

• Water molecules are attracted to each other – opposite poles line up

Molecular Forces in Water• H2O molecules held together by

covalent bonds (intra-molecular, solid lines)

• and hydrogen bonds (inter –molecular,

dashed lines)

Hydrogen Bonding

the attraction of hydrogen atoms in one molecule to an electronegative atom on

another molecule

Water: The Hydrogen Bonding Champion

• Water will form hydrogen bonds with any molecule with O – H bonds– alcohols– proteins– nucleic acids– carbohydrates

Water: Physical Properties Revisited

• hydrogen bonds hold H2O molecules together and result in

–H2O being a liquid at room temp–high boiling point

Ice Floats• Solid H2O is less dense than liquid

H2O

• liquid H2O increases in density as it cools

• When it hits maximum density @ 4o C volume begins to expand and density decreases

Ice FloatsWhy does this happen?

• below 4o C water molecules begin to form an open arrangement due to hydrogen bonding

• volume expands and• density decreases

Surface Tension• surface tension – force needed to

overcome intermolecular attraction and break through the surface of a liquid

• high surface tension = high resistance

• H2O has high surfacetension

CapillarityHow does a liquid travel UP a tube?• capillarity – competition between– inter-particle attractive forces– attractive forces between the liquid

and the tube

Water: Earth’s Thermostat

• Specific heat is the amount of heat in joules needed to raise the temperature of 1 g substance 1oC.

• Water has a high specific heat

The Dissolving ProcessHow does water dissolve ionic

substances?• polar water molecules surround the

ions• the + hydrogen pulls on the – ions • the - oxygen pulls on the + ions

• this is calleddissociation

The Dissolving ProcessHow water dissolves a covalent

substance:

• Water forms hydrogen bonds withthe O-H group of covalent molecule

• The hydrogen bond overcomes the intermolecular forcesof the sugar molecule

Like Dissolves LikeHow can you predict whether a

substance will dissolve in another substance?

• like dissolves like• solvent and solute

must have similarities

Like Dissolves Like

• How is water like an ionic compound?

• ionic compounds have charged (- & +) ions

• water is polar (has partially charged ends)

Like Dissolves Like

• How is water like sugar?

• both have O-H groups and form hydrogen bonds

Concentrated Versus Dilute

• weak and strong are used to describe acids and bases

• chemists use concentrated and dilute to describe solutions

Unsaturated Versus Saturated

• unsaturated - a solution that has less than the maximum amount of solute dissolved

• saturated – a solution that holds the maximum amount of dissolved solute

Unsaturated Versus Saturated

supersaturated – a solution that contains MORE than the maximum amount of dissolved solute --- unstable

Molarity• molarity - # of moles of SOLUTE

per liter of SOLUTION

• used to determine the exact concentration of a solution

• abbreviated --- M (= moles/liter)

MolarityWhen making a solution you need to

know:– the concentration– the amount of solute– total volume of solution needed

Ex: How would you prepare 5.0 L of a 0.15 M sodium chloride solution?

MolarityEx: How would you prepare 5.0 L of a 0.15 M

sodium chloride solution?

5.0 L solution X 0.15 mol NaCl X 58.5 g NaCl 1 1.0 L solution 1 mol NaCl

= 43.9 g NaCl

So you need to mix 43.9 g of NaCl with enough water to equal 5.0 L of solution

MolarityEx: How would you prepare 2.5 L of a 0.80 M KNO3 solution?

2.5 L solution X 0.80 mol KNO3 X 101 g KNO3

1 1.0 L solution 1 mol KNO3

= 202 g KNO3

So you need to mix 202 g of KNO3 with enough water to equal 2.5 L of solution